1. Field of the Invention
Intake and exhaust performance of an internal combustion engine can be improved by increasing what is known as a "valve area", i.e., the total area occupied by intake and exhaust valves in a cross-sectional area of a combustion chamber of each cylinder of the engine. For this reason, a multi-valve internal combustion engine having such intake and exhaust valves is typically provided with more than one exhaust valve and more than one intake valve. To improve the output of such a multi-valve internal combustion engine, however, it is necessary for the intake and exhaust valves themselves to be small in size and light in weight so that they can properly follow and operate throughout various speeds of rotation of the engine. This is particularly important in range of high engine operating speeds.
Such a multi-valve internal combustion engine, known from, for instance, a Japanese patent application entitled "Four Cycle Engine," filed on Dec. 28, 1981, and published as Japanese Unexamined Patent Publication No. 58-113,512 on Jul. 6, 1983, has a drawback in that tumbling, i.e., turbulent, air motion, which generally contributes to stimulating fuel mixture combustion, is interrupted. Considering, as an example, an internal combustion engine having three intake valves for one cylinder, three intake ports are typically arranged so that an intermediate, or center, intake port is closer to a center axis of the cylinder than both side intake ports disposed on opposite sides of the center intake port with respect to the center axis. Such an arrangement of intake ports is usually accompanied by a positional irregularity of valve stems of the intake valves. To eliminate valve drive system complexity due to the positional irregularity of valve stems, the valve stem of the intake valve for the center intake port is inclined at an angle with respect to the center axis of the cylinder which is smaller than angles at which the valve stems of the intake valves for the side intake ports are inclined so that top ends of all the valve stems of the intake valves are on a straight line, parallel to an axis of rotation of a cam shaft of the valve drive system. For this purpose, it is necessary for the center intake port to be inclined at an angle with respect to the center axis of the cylinder which is relatively small as compared to an angle or angles at which the side intake ports are inclined. In the intake system, even if the side intake ports direct a fuel mixture so that it flows toward an inner surface of the cylinder to cause the fuel mixture to tumble or bounce on the inner surface of the cylinder and the top surface of a piston, thereby providing the fuel mixture with a tumbling motion, the tumbling motion of the fuel mixture introduced through the side intake ports will be overwhelmed by a fuel mixture flow introduced through the center intake port which is bounced by the piston and directed against the fuel mixture flow introduced through the side intake ports. In other words, in the conventional intake system, tumbling air motion has not been taken into design considerations.